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[PATCH] Fix displaced stepping for remote targets


Hi,

While debugging a remote target that supports hardware single-stepping with a GDB that supports displaced stepping, i've ran into the following problem...

When reaching a breakpoint, GDB should take care of relocating the instruction underneath that breakpoint to the scratch space where it will be executed out-of-line.

If a target supports hw single-stepping for displaced stepping, GDB should just send a vCont;s packet to tell the target to step a single instruction. In my case, GDB was always sending a vCont;c instead.

I've tracked it down to infrun.c:resume, where we have this check:

      if (gdbarch_cannot_step_breakpoint (gdbarch))
        {
          /* Most targets can step a breakpoint instruction, thus
             executing it normally.  But if this one cannot, just
             continue and we will hit it anyway.  */
          if (step && breakpoint_inserted_here_p (aspace, pc))
            step = 0;
        }

My target can't step breakpoints and, if we're doing displaced stepping, it's because we're trying to step off a breakpoint, thus breakpoint_inserted_here_p returns true, and we disable single-stepping by setting step to 0.

It seems to me we need to update the PC prior to calling breakpoint_inserted_here_p since the displaced stepping machinery adjusted the old PC to point to the space in the scratch area. That way we can properly command the target to step the displaced instruction and we can check for breakpoints at the real execution place.

The following patch fixes this by pushing the if block further down in the code and taking care of updating PC if displaced stepping is being used.

I've regtested this on x86 and everything looks OK. This also makes GDB send vCont;s now.

Ok?

Luis
2012-03-19  Luis Machado  <lgustavo@codesourcery.com>

	* infrun.c (resume): Change order of statements and
	update PC to the real PC in case of displaced stepping.

Index: HEAD-git/gdb/infrun.c
===================================================================
--- HEAD-git.orig/gdb/infrun.c	2012-03-19 20:40:57.589621232 -0300
+++ HEAD-git/gdb/infrun.c	2012-03-19 22:12:08.729898472 -0300
@@ -1884,15 +1884,6 @@ a command like `return' or `jump' to con
 	  resume_ptid = inferior_ptid;
 	}
 
-      if (gdbarch_cannot_step_breakpoint (gdbarch))
-	{
-	  /* Most targets can step a breakpoint instruction, thus
-	     executing it normally.  But if this one cannot, just
-	     continue and we will hit it anyway.  */
-	  if (step && breakpoint_inserted_here_p (aspace, pc))
-	    step = 0;
-	}
-
       if (debug_displaced
           && use_displaced_stepping (gdbarch)
           && tp->control.trap_expected)
@@ -1902,12 +1893,25 @@ a command like `return' or `jump' to con
           CORE_ADDR actual_pc = regcache_read_pc (resume_regcache);
           gdb_byte buf[4];
 
+	  /* Update pc to reflect the new address from which we will execute
+	     instructions due to displaced stepping.  */
+	  pc = actual_pc;
+
           fprintf_unfiltered (gdb_stdlog, "displaced: run %s: ",
                               paddress (resume_gdbarch, actual_pc));
           read_memory (actual_pc, buf, sizeof (buf));
           displaced_step_dump_bytes (gdb_stdlog, buf, sizeof (buf));
         }
 
+      if (gdbarch_cannot_step_breakpoint (gdbarch))
+	{
+	  /* Most targets can step a breakpoint instruction, thus
+	     executing it normally.  But if this one cannot, just
+	     continue and we will hit it anyway.  */
+	  if (step && breakpoint_inserted_here_p (aspace, pc))
+	    step = 0;
+	}
+
       /* Install inferior's terminal modes.  */
       target_terminal_inferior ();
 

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